The phylogenetic significance of crustacean optic neuropils and chiasmata: A re‐examination

Recent molecular data challenge the traditional hypotheses of arthropod phylogeny founded on morphologic characters. In this discussion, the structure of the visual systems in Pterygota (Hexapoda) and Decapoda (Malacostraca, Crustacea) is an important argument. Although many components of their visual systems depict structural homology, differences exist between Pterygota/Decapoda on the one side and Branchiopoda (Entomostraca) on the other in that the latter do not have a third optic neuropil or optic chiasmata. Therefore, the goals of the current study were to explore whether the third optic neuropils in Pterygota and Decapoda are homologous, to examine the formation of the first two optic neuropils and the chiasmata in Crustacea, and to compare these processes with Pterygota. For this purpose, five species of entomostracan and malacostracan crustaceans were analyzed by examination of serial sections, fluorescence labeling with phallotoxins, and anti-histamine immunohistochemistry. We found that the chiasmata of Decapoda and Pterygota are characterized by striking similarities regarding both the level of individually identifiable classes of neurons and ontogenetic mechanisms, which are clearly different from those in Branchiopoda. Furthermore, the third optic neuropil of Decapoda and Pterygota, the lobula, shares an ontogenetic protocerebral origin and an innervation by corresponding sets of histamine-immunoreactive neurons, suggesting homology of the lobula in these two groups. In conclusion, the characteristics of the visual system are in conflict with the traditional classification of Arthropoda. Instead, they support a sister-group relationship of Hexapoda and Malacostraca, as suggested by some of the molecular studies.